This invention relates to an improvement of a turbine control system which controls a turbine provided with a plurality of control valves in an electro-hydraulic control method.
In general, in the control system of a steam turbine, when steam of high temperature and high pressure is introduced from a steam generating device to the steam turbine through a plurality of steam control valves, the flow-rate of the steam is controlled by operating these steam control valves thereby to control the speed and output of the turbine. Especially in starting the turbine, all of the steam control valves are subjected to a so-called "throttle governing control" in which the steam control valves completely closed are gradually opened until the output of the turbine reaches a predetermined value, and thereafter, to a so-called "nozzle governing control" in which the steam control valves are successively fully opened according to the output of the turbine. For this purpose, that is, in order to switch the throttle governing operation over to the nozzle governing operation, conventional steam turbines are provided with control systems for changing the opening degrees of the steam control valves.
In such control system, in order that the steam control valves can provide the desired operating performance of the turbine during its operation, the "opening degree"-"steam flow-rate" characteristic of each steam control valve is corrected with the aid of a main control flow-rate request signal delivered from a speed control section or a load control section so that the opening-degree of each of the steam control valves is changed through a valve position control section to a suitable value.
However, it should be noted that even if one and the same main control flow-rate requesting signal is applied to the turbine, the opening degree characteristic of each steam control valve in the throttle governing operation is different from that in the nozzle governing operation. Accordingly, when the operating condition of the turbine is quickly changed from the throttle governing control condition to the nozzle governing control condition by changing the valve opening degrees, some of the steam control valves abruptly increase their opening-degrees and the steam of high temperature and high pressure rapidly flows into the turbine therethrough, thereby imparting thermal shocks to the nozzle box, the turbine casing, etc., which may cause serious damage to the turbine.
In order to overcome such difficulty accompanying the conventional steam turbines a technique according to U.S. Pat. No. 3,688,095 (corresponding to Japanese Pat. No. 627,126) has been proposed. In this technique, an analog control circuit includes contact means, and therefore its circuit is inevitably intricate, and during the valve opening degree changing operation, the above-described thermal shock is liable to be caused depending on the offsetting conditions of an amplifier and other elements included therein. In this technique, no variation is caused in its steam flow-rate before and after the valve opening degree changing operation; however, during this valve opening degree changing operation the steam flow-rate is varied because no control to keep the steam flow-rate constant is provided. That is, it is impossible to eliminate the variable output of the turbine by the proposed technique.